Extended wavelength digital alloy NBN detector

ABSTRACT

A strain-balanced photodetector is provided for detecting infrared light at an extended cutoff wavelength in the range of 4.5 μm or more. An InAsSb absorber layer has an Sb content is grown in a lattice-mismatched condition to a GaSb substrate, and a plurality of GaAs strain-compensating layers are interspersed within the absorber layer to balance the strain of the absorber layer due to the lattice mismatch. The strain-compensation layers allow the absorber to achieve a thickness exhibiting sufficient absorption efficiency while extending the cutoff wavelength beyond that possible in a lattice-matched state. Additionally, the strain-compensation layers are sufficiently thin to be substantially quantum-mechanically transparent such that they do not substantially affect the transmission efficiency of the absorber. The photodetector is preferably formed as a majority carrier filter photodetector exhibiting minimal dark current, and may be provided individually or in a focal plane array.

FIELD OF THE INVENTION

The present invention relates in general to photodetectors, and inparticular to infrared photodetectors with a majority carrier filterstructure.

BACKGROUND OF THE INVENTION

A photodetector sensitive to the infrared wavelengths of light is alsoknown as an infrared detector. Infrared detectors are used in a widevariety of applications, and in particular are used as thermal detectionfor surveillance, tracking, night vision, search and rescue, nondestructive testing and gas analysis. Typically, an infrared detector isformed as a device consisting of an array, usually rectangular, ofinfrared light-sensing photodetectors disposed at the focal plane of animaging lens. Such a detector is commonly referred to as a focal planearray (FPA).

Infrared covers a broad range of wavelengths, and many materials areonly sensitive to a certain range of wavelengths. As a result, theinfrared band is further divided into sub-bands such as near infrareddefined conventionally as 0.75 to 1.0 μm; short-wavelength infrared(SWIR) defined conventionally as 1.0 to 3.0 μm; mid-wavelength infrared(MWIR) defined conventionally as 3 to 5 μm; and long-wavelength infrared(LWIR) defined conventionally as 8 to 14 μm. Infrared in the range of 5to 8 μm is not transmitted well in the atmosphere and thus manymid-wavelength infrared detection applications operate within the 3 to 5μm atmospheric window portion of the MWIR band.

Infra-red photon detectors are often produced using InSb and HgCdTe p-njunction diodes. However, these thermal detectors require cooling tocryogenic temperatures of around 77 K, which is complex, energy andvolume consuming, and costly. The cryogenic temperatures are primarilyused to reduce the dark current generated in the p-n junction diode inthe bulk and at the surface by Shockley Reed Hall (SRH) generation,among other effects.

Photodetectors comprising a photo-absorbing layer, a barrier layer, anda contact layer have overcome many disadvantages of priorphotodetectors, including mid-wavelength infrared detectors. A new classof photodetectors employing majority carrier filter principles isdescribed in U.S. Pat. No. 7,687,871 to Maimon, filed Mar. 19, 2006, theentire contents of which are hereby incorporated by reference. Thesemajority carrier filter photodetectors are often referred to as nBndetectors although the majority carrier filter principles may beemployed using a variety of doping arrangements.

Photodetectors have been produced that are sensitive to a targetwaveband and that comprise a photo-absorbing layer preferably exhibitinga thickness of between one and two times the optical absorption length.These photodetectors may be comprised of an n-doped photo-absorbinglayer, a barrier layer, and an n-doped contact layer. Other dopings maybe used, such as p-doped photo-absorbing and contact layers as describedby Maimon. These detectors may use an absorber to convert the incomingradiation into minority carriers which are collected to generatephotocurrent. These detectors may use a barrier layer whose minoritycarrier band edge lines up with the absorber minority carrier band edgeso that carriers can be collected. The majority carrier band edge of thebarrier is well above the contact or absorber band edge such thatmajority carriers are blocked or filtered, thus producing the functiondescribed by the term “majority carrier filter.” The barrier layerexhibits a thickness sufficient to prevent tunneling of majoritycarriers from the photo-absorbing layer to the contact layer, and abarrier in the majority carrier energy band sufficient to block the flowof thermalized majority carriers from the photo-absorbing layer to thecontact layer. The barrier layer does not significantly block minoritycarriers when appropriate bias voltage is applied.

In particular, for an n-doped photo-absorbing layer the heterojunctionbetween the barrier layer and the absorbing layer is such that there issubstantially zero valence band offset, i.e. the band gap differenceappears almost exclusively in the conduction band offset. For a p-dopedphoto-absorbing layer the heterojunction between the barrier layer andthe absorbing layer is such that there is substantially zero conductionband offset, i.e. the band gap difference appears almost exclusively inthe valence band offset. Advantageously, these photodetectors can beoperated with minimal to no depletion layer, and thus the dark currentis significantly reduced. Furthermore, passivation is not required inarrayed photodetector elements as the barrier layer further functions toachieve passivation.

The specific materials used to produce a majority carrier filter are notcritical so long as the valance and conduction bands are configured asdescribed above. However, the materials should be selected to producethe valence and conduction band relationships discussed above. Thebarrier layer may comprise any suitable material such as one of AlSb,AlAsSb, GaAIAsSb, AlPSb, AlGaPSb and HgZnTe. Similarly, thephoto-absorbing layer may be desirably constituted of one of n-dopedInAs, n-doped InAsSb, n-doped InGaAs, n-doped InGaAsSb, n-doped Type IIsuper lattice InAs/InGaSb and n-doped HgCdTe. The contact area may beconstituted of one of InAs, InGaAs, InAsSb, InGaAsSb, Type II superlattice InAs/InGaSb, HgCdTe and GaSb. Alternatively, the photo-absorbinglayer and/or contact layer may be p-doped. The contact layer and thephoto-absorbing layer may exhibit substantially identical compositions.In the case where the photo-absorbing layer and contact layer have thesame doping type but two different bandgaps, two-color operation can beachieved by reversing the bias voltage so that the photons absorbed inthe “contact” layer are now collected in the “absorbing” layer. The biascan be alternatingly reversed so as to collect photons within twodifferent radiation bands, corresponding to photons collected when thephotodetector is forward-biased and when the photodetector isreverse-biased, respectively. For backside-illuminated focal planearrays the top or “contact” layer should have the smaller bandgap inorder to absorb the longer radiation band.

For a photo-absorbing material made from semiconductor materials, theabsorption cutoff wavelength of the photo-absorbing material isgenerally determined by the composition of the semiconductor, but may belimited by dislocations in the molecular structure of the semiconductorlattice. The absorber material may be grown by liquid phase epitaxy(LPE), molecular beam epitaxy (MBE), metal-organic chemical vapordeposition (MOCVD), or other methods known to those skilled in the arton substrate materials such as InSb, GaSb, InAs, InP, etc. However, toavoid substantial dislocations in the absorber material, the absorbershould be a composition that has a crystal lattice constant similar tothat of the substrate material. If the lattice constant of the absorberalloy does not match the lattice constant of the substrate material, thestrain on the composition of the absorber due to the mismatch betweenthe absorber and substrate lattice structures will increase as theabsorber is grown, and will ultimately result in dislocations in thelayers of the absorber lattice once the absorber exceeds a criticalthickness.

For practical purposes, to grow an absorber with sufficient thicknesssuch that the absorber has reasonable quantum efficiency, the latticeconstant of the absorber must very closely match the lattice constant ofthe substrate upon which the absorber material is grown. Thisrequirement that the absorber materials be lattice-matched to thesubstrate material effectively limits the absorption cutoff wavelengthsof the photodetector to specific values. For example, an absorbercomprising InAs(0.9)Sb(0.1), when lattice-matched to a substratecomprising GaSb and having a lattice constant of 6.09 A, exhibits anassociated cutoff wavelength of 4.2 μm. However, this cutoff wavelengthfalls well short of the upper end of the 3 to 5 μm range for MWIRdetection applications operating within the atmospheric window of theinfrared band. Since there is much more infrared flux in the longerwavelength it is desirable to absorb the full MWIR band when the highestsensitivity for the photodetector is desired.

FIG. 2 shows the behavior of unstrained band-edge discontinuities as afunction of the lattice constant. The unstrained band alignments of anytwo lattice-matched alloys can be determined by noting the relativeposition of their band edges in FIG. 2. For strained conditions, themechanical strain fields alter the behavior of the electronicwavefunctions, thereby changing the valence and conduction bands, butthe general behavior is similar. Highly ordered atomic transitions occurbetween lattice-matched semiconductor heterostructures with relativelylittle atomic and electronic reconstruction. However, in alattice-mismatched condition defects occur in the crystal structure ofthe absorber when the absorber lattice dislocates to relieve the excessstrain resulting from the lattice mismatch between the absorber and thesubstrate. These crystalline defects directly effect the operatingcharacteristics of the photodetector device, degrading the quality, e.g.the radiative efficiency and thermal noise, of the semiconductor.

There are limited substrates available for crystal growth, usually thebinary materials like InAs or GaSb shown in FIG. 2 although other alloysmay be used. Conventionally, the only cutoff wavelengths available comefrom alloys whose lattice constant match the substrate. It is desirablefor design versatility in various infrared sensing applications to beable to achieve cutoff wavelengths other than and beyond those oflattice-matched systems. Sometimes it is possible to grow quaternariesto raise or lower the band gap, for example, by adding aluminum ornitrogen to the absorber material. However, this approach can result inpoor absorber material quality or a complicated growth processes.

Additionally, despite the potential for lattice defects, photodetectorswith highly strained absorbers have been grown to achieve an extendedcutoff frequency. For example, the maximum wavelength normallyachievable with a highly strained absorber on a substrate comprisingGaSb is 4.5 μm or less. However, the high strain resulting from thisapproach limits the absorber thickness and material quality.Superlattices using InAs/GaSb have been used to create an effectivetunable bandgap in the GaSb system. However, the complicated growth andthe band edges of the superlattice system relative to the barrier havemade it difficult to achieve MWIR barrier detectors with superlatticeabsorbers.

It is desirable for photodetectors, and in particular majority carrierfilter type photodetectors with reduced dark current, to operate inwavelengths not accessible using materials in the lattice-matchedcondition. Aspects of the present invention relate generally to solvingthe problems associated with photodetectors, in particular majoritycarrier filter type photodetectors, where absorption cutoff wavelengthsare limited by absorber/substrate lattice-matching requirements, suchthat photodetectors exhibiting cutoff wavelengths beyond conventionalcutoff wavelengths can be realized. It is desirable to operate inwavelengths not accessible in the lattice-matched condition and toextend the wavelength out to 5.0 μm or further to take full advantage ofthe 3 to 5 μm atmospheric transmission portion of the MWIR band. Thepresent invention has been developed in view of these considerations,and therefore it is an object of the invention to provide aphotodetector with an extended cutoff while also maintaining the bandlineups at the junctions of the layers of majority carrier filterdetector.

SUMMARY OF THE INVENTION

The present invention relates to a photodetector, in particular amajority carrier filter photodetector, comprising a photo-absorbingregion comprising a plurality of strained photo-absorbing layers of afirst alloy and a plurality of strain-compensating layers of a secondalloy interspersed between the photo-absorbing layers. The opticalabsorption is in the photo-absorbing layers, whose spectral absorptionproperties are determined by the photo-absorbing layer composition.

According to aspects of the present invention, the material forming theplurality of strain-compensation layers exhibit a substantiallydifferent lattice constant than that of both the substrate and theremaining absorber material. These thin layers have an aggregate strainsubstantially equal to the aggregate strain of the photo-absorbinglayers but of opposite sign, thereby compensating for the strain of theabsorber material. This allows for the growth of sufficiently thickaggregate photo-absorbing layers which do not contain dislocations inthe lattice structure of the semiconductor alloy comprising thephoto-absorbing layers. The resultant strain-compensated structure ofthe photodetector allows for the use of an absorber alloy exhibiting adifferent lattice constant than the lattice constant of the substratewhile exceeding the critical thickness at which the lattice structurenormally dislocates. The relationship between the critical thicknesswith respect to increasing strain and eventual dislocations betweenlattice-mismatched materials is described, for example, in J. W.Matthews and A. E. Blakeslee, J. Cryst. Growth 27, 118 (1974), theentire contents of which are incorporated herein by reference.

More specifically, according to aspects of the present invention, thestrain-compensation layers may be grown alternately with thephoto-absorber alloy using epitaxial techniques. The strain-compensationlayers are grown sufficiently thin such that they are substantiallytransparent in quantum-mechanical effect and do not significantly affectthe spectral properties of the overall photo-absorbing region. Thisallows for growth of a photo-absorbing region sufficiently thick to havereasonable quantum efficiency and without exhibiting the dislocationdefects that normally would be caused by excessive strain within theabsorber alloy. This allows the photodetector to operate beyond thecutoff wavelength of a photodetector with an absorber layer comprising atraditional alloy lattice-matched to the substrate. Similarly one canachieve shorter cutoff wavelengths than the lattice matched alloy, ifdesirable.

According to aspects of the present invention, the strain-compensationlayers are sufficiently thin to be effectively quantum-mechanicallytransparent. The plurality of strain-compensating layers can beinterspersed between the plurality of photo-absorbing layers at aperiodic interval such that the strain-compensation layers aresubstantially transparent to quantum waveforms. Therefore the cutoffwavelength can be extended while also maintaining the band lineups atthe junctions of the layers of a majority carrier filter photodetector,as well as maintaining the reduced dark current characteristics ofmajority carrier filter photodetector. These and other advantages of thepresent invention will become evident to those skilled in the art.

In an exemplary embodiment of the present invention, the photo-absorbinglayers are comprised of InAsSb alloy, the strain-compensation layers arecomprised of GaAs alloy, and the substrate is comprised of GaSb alloy.By interspersing the GaAs strain-compensation layers between thephoto-absorbing layers, the antimony content in the InAs_(x)Sb_(1-x),alloy can be increased such that the photodetector may exhibit a cutoffwavelength exceeding the lattice-matched 4.2 μm and preferably extendingto 5.0 μm or more.

According to alternative embodiments of the present invention, thephoto-absorbing layers may be comprised of InGaAsSb alloys.Additionally, the strain-compensation layers may be alternativelycomprised of InSb alloy. The substrate may be comprised of InAs binary.In addition to the alloys described herein, one of ordinary skill in theart will readily appreciate that many other alloys of thin straincompensating layers may be used without deviating from the scope of theinventive concept described herein.

According to certain embodiments of the present invention, the latticeconstant of the photo-absorbing layers is larger than the substrate suchthat each photo-absorbing layer is compression-strained and eachstrain-compensating layer is tensile-strained. In alternativeembodiments, the lattice constant of the photo-absorbing layers issmaller than the substrate such that each photo-absorbing layer istensile-strained and each strain-compensating layer iscompression-strained.

According to a further embodiment of the present invention, the contactlayer may comprise a plurality of photo-absorbing layers and a pluralityof strain-compensating layers interspersed between the plurality ofphoto-absorbing layers, wherein the photo-absorbing layers aresubstantially lattice-mismatched to the barrier layer, and thestrain-compensating layers are interspersed between the photo-absorbinglayers so as to substantially compensate for a mechanical strain of thephoto-absorbing layers caused by the lattice-mismatched condition.

In a further embodiment of the present invention, a strain-balancedtwo-color photodetector is provided, which can detect infrared lightwithin two radiation bands each having a different cutoff wavelength.According to the features of this embodiment, at least one of a firstlayer (photo-absorbing layer) and a second layer (contact layer) locatedabove the barrier layer is grown substantially lattice-mismatched to thesubstrate and/or the barrier layer. Either or both of the first andsecond layers may comprise a plurality of strain-compensating layersinterspersed between a plurality of photo-absorbing layers so as tosubstantially compensate for a mechanical strain of the photo-absorbinglayers caused by the lattice-mismatched condition. The first layer canexhibit a first cutoff wavelength and the second layer can exhibit asecond cutoff wavelength, wherein the first cutoff wavelength can beshorter than the second cutoff wavelength.

According to certain embodiments of the two-color photodetector of thepresent invention, the first layer can comprise an alloy of InAsSbsubstantially lattice-matched to GaSb, and the second layer can comprisea plurality of tensile-strained strain-compensating layers interspersedbetween a plurality of compressive-strained photo-absorbing layerscomprised of InGaAsSb. Additionally, the strain-compensating layers inthe second layer can be comprised of GaAs.

In an alternative embodiment of the two-color photodetector of thepresent invention, the first layer can comprise a plurality ofcompressive-strained strain-compensating layers comprised ofInAs_(w)Sb_(1-w) interspersed between a plurality of tensile-strainedphoto-absorbing layers comprised of In_(x)Ga_(1-x)As_(y)Sb_(1-y), andthe second layer can comprise an alloy of InAsSb substantiallylattice-matched to GaSb. Additionaly, the strain-compensating layers inthe first layer can be comprised of InSb.

In an alternative embodiment of the two-color photodetector of thepresent invention, the first layer can comprise a plurality ofcompressive-strained strain-compensating layers comprised ofInAs_(w)Sb_(1-w) interspersed between a plurality of tensile strainedphoto-absorbing layers comprised of In_(x)Ga_(1-x)As_(y)Sb_(1-y). Thestrain-compensating layers of the first layer can be comprised of InSb,and the strain-compensating layers of the second layer can be comprisedof GaAs.

In an alternative embodiment of the two-color photodetector of thepresent invention, the first layer can comprise a plurality ofphoto-absorbing layers comprised of InAsSb and a plurality ofstrain-compensating layers comprised of GaAs, and the second layercomprises a plurality of photo-absorbing layers comprised of InGaSb anda plurality of strain-compensating layers comprised of InSb.

According to further aspects of the two-color photodetector the presentinvention, the substrate can comprise GaSb or InAs, and the barrierlayer can comprise AlAsSb or AlGaAsSb.

In a further embodiment of the present invention, a photodetector isprovided where the minority carrier bandedge of at least one of thefirst and second layers can be graded vertically by varying the alloycomposition in a direction toward the barrier layer. A plurality ofvarying strain-compensating layers can be interspersed within a gradedlayer such that the lattice structure is substantially prevented fromdislocating. In certain embodiments of the present invention, theinterval between the strain-compensating layers in the graded layer canbe gradually varied in the direction toward the barrier layer in orderto balance the varying strain within the graded layer. In otherembodiments of the present invention, the thickness of thestrain-compensating layers in a graded layer can be gradually varied inthe direction toward the barrier layer in order to balance the varyingstrain within the graded layer. Alternatively, both of the thickness andperiodicity of the strain-compensation layers may be varied in order tobalance the varying strain within a graded layer.

According to a further embodiment of the present invention, a focalplane array with an extended cutoff wavelength may be provided byforming a two-dimensional matrix of a plurality of photodetectors withstrain-balanced structures formed according to any of the aforementionedembodiments.

In an additional embodiment, a method of forming a strain-balancedextended-wavelength photodetector for a desired cutoff wavelength isprovided. According to the method, a desired cutoff wavelength for thephotodetector is selected. Then, a mole fraction is determined for theabsorber alloy corresponding to the desired cutoff wavelength to beexhibited by the absorber layer. A layer thickness ratio between athickness of the strain-compensation layers and the photo-absorbinglayers within the absorber is determined, wherein the ratio issufficient to achieve strain balancing of the photo-absorbing layers forthe determined mole fraction. Next, a periodic interval is selected forproviding the strain-compensation layers within the absorber at thethickness corresponding to the determined layer thickness ratio suchthat the strain-compensation layers are substantially electricallytransparent to minority carriers. The photodetector absorber layer isthen grown by alternatingly growing on the substrate a plurality ofphoto-absorbing layers of a first alloy and the plurality ofstrain-compensation layers of a second alloy at the determined periodicinterval such that the strain-compensating layers are substantiallyquantum-mechanically transparent while compensating for a mechanicalstrain of the photo-absorbing region on a substrate in alattice-mismatched condition.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomereadily apparent from this detailed description to those skilled in theart.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given herein below and the accompanying drawings,which are given by way of illustration only and thus are not limitativeof the present invention, and wherein:

FIG. 1A shows a schematic cross-section view of an exemplary embodimentof a photodetector according to the present invention.

FIG. 1B shows a schematic cross-section view of an exemplary embodimentof a photodetector according to the present invention, wherein bothelectrodes are disposed on the top side of the photodetector.

FIG. 2 shows the bandgap energies between the valence and conductionband edges for a III-V alloy system.

FIG. 3 shows the bandgap energies between the valence and conductionband edges for a GaSb substrate, InAsSb photo-absorbing layers, and GaAsstrain-compensating layers, according to the exemplary embodiments ofFIGS. 1A and 1B.

FIG. 4 shows the conduction and valence energy bands of thephoto-absorbing layers and the strain-compensating layers for anexemplary embodiment of the present invention.

FIG. 5 shows the band edge energies versus growth direction for anexemplary embodiment of the present invention comprising InAsSbphoto-absorbing layers and GaAs strain-compensating layers.

FIG. 6 shows the relationship between the compressive strain and tensilestrain of the respective InAsSb photo-absorbing layers and GaAsstrain-compensating layers with respect to a GaSb substrate, in theexemplary embodiments of the present invention.

FIG. 7 shows an exemplary method of designing an extended wavelengthphotodetector to have strain-compensation characteristics that achieve aparticular cutoff wavelength while maintaining reasonable quantumefficiency.

FIG. 8 shows the relationship between the Sb mole fraction and thecutoff wavelength for InAs(1−x)Sb(x).

FIG. 9 shows the relationship between the Sb mole fraction and thethickness ratio between the InAsSb photo-absorbing layers and the GaAsstrain-compensating layers in the exemplary embodiments of FIGS. 1A and1B.

FIG. 10 shows the relationship between the thickness of the potentialbarrier or well for the strain-compensating layers, the height of thepotential barrier or depth of the well relative to the absorber bandedge, and the energy of a particle relative to the absorber band edge,according to aspects of the present invention.

FIG. 11 shows the transmission characteristics for electrons (top),light holes (middle), and heavy holes (bottom) as a function of thethickness of the strain-compensating layers, according to calculationsbased on expressions (4), (5), and (6) as described herein.

FIG. 12 shows a plot of the mini-bands along with the band edges alongthe growth direction, according to aspects of the present invention.

FIG. 13 shows an example of band edge alignment between the holemini-bands and the barrier valence band edge, according to aspects ofthe present invention.

FIG. 14 shows a schematic cross-section view of another embodiment ofthe present invention in which a focal plane array is formed as a matrixof strain-balanced photodetectors formed according to the embodiments ofthe present invention.

FIG. 15A shows a schematic top view of the focal plane array accordingto the embodiment of FIG. 14.

FIG. 15B shows a schematic top view of an alternate embodiment of thefocal plane array of FIG. 14, wherein both electrodes are disposed onthe topside of the photodetector according to the embodiment of FIG. 1B.

FIG. 16 shows the band edge energies between the valence and conductionband edges for an InAs substrate, InAsSb photo-absorbing layers, andGaAs strain-compensating layers, according to an alternative embodimentof the present invention.

FIG. 17 shows the bandgap energies between the valence and conductionband edges for a GaSb substrate, InGaAs photo-absorbing layers, and InSbstrain-compensating layers, according to an alternative embodiment ofthe present invention.

FIG. 18 shows a schematic cross-section view of a photodetectoraccording to a further embodiment of the present invention in which thecontact layer region is strain-balanced by alternatingstrain-compensating layers and photo-absorbing layers.

FIG. 19 shows an example of band edge alignment between the holemini-bands and the barrier valence band edge of a strain-balancedtwo-color photodetector exhibiting two different cutoff wavelengths,according to an alternative embodiment of FIG. 18.

FIG. 20 shows a majority carrier filter photodetector according tofurther embodiments of the present invention where the photo-absorberbandgap has been graded by increasing the Sb content of the absorber inthe direction toward the barrier so as to drive minority carrierstowards the barrier, wherein the graded photo-absorber isstrain-balanced according to a further embodiment of the presentinvention.

The drawings will be described in detail in the course of the detaileddescription of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention refers to theaccompanying drawings. The same reference numbers in different drawingsidentify the same or similar elements. Also, the following detaileddescription does not limit the invention. Instead, the scope of theinvention is defined by the appended claims and equivalents thereof.

FIG. 1A shows a schematic cross-section view of an exemplary embodimentof a photodetector 10 of the present invention. Photodetector 10comprises a semiconductor substrate 12 on which a plurality of III-Vcompound semiconductor layers are epitaxially grown to form aphoto-absorbing region 14, a barrier layer 16, and a contact layer 18.The photo-absorbing region 14 (i.e. absorber, photo-absorber, absorberlayer) comprises a plurality of strain-compensating layers 22 which arealternatingly interspersed between a plurality of photo-absorbing layers20. In addition, a top electrode 24 is located above the contact layer18 and a bottom electrode 26 is located below the substrate 12. In analternate preferred embodiment shown in FIG. 1B, a via 34 is etched inthe barrier layer 16 down to the top surface of the photo-absorbingregion 14, and the bottom electrode 26 is located on the absorber layertop surface 30 through the via 34.

In the exemplary embodiments of FIGS. 1A and 1B, the photo-absorbinglayers 20 comprise indium arsenide antimonide (InAsSb) and thestrain-compensating layers 22 comprise gallium arsenide (GaAs). Thesubstrate 12 comprises gallium antimonide (GaSb). In other embodimentsof the present invention, the photo-absorbing layers 20 can compriseindium gallium arsenide (InGaAs). In other embodiments of the presentinvention, the strain-compensating layers 22 can comprise indiumantimonide (InSb). In further embodiments of the present invention, thesubstrate can comprise indium arsenide (InAs). In addition to thesealloys, one of ordinary skill in the art will readily appreciate thatother alloys may be used without deviating from the scope of theinventive concept described herein.

In the embodiments of FIGS. 1A and 1B, the composition of the InAsSbphoto-absorbing layers 20 includes a sufficient mole fraction ofantimony (Sb) content such that the photodetector absorbs incidentinfrared light 50 at a predetermined cutoff wavelength longer than theconventional lattice-matched wavelength of 4.2 μm. Preferably, thecomposition of the InAsSb photo-absorbing layers 20 is sufficient toabsorb infrared light 50 at wavelengths of 5.0 μm or more in order totake full advantage of the atmospheric region of the MWIR band. As usedherein, the term “cutoff wavelength” refers to the wavelength on along-wavelength side of a response curve of the photodetector 10 forwhich the response of the photodetector 10 is reduced by 50 percent fromthe peak response of the photodetector. A short cutoff wavelength of thephotodetector 10, for which the response of the photodetector 10 isreduced by 50 percent from the peak response on a short-wavelength sideof the response curve of the photodetector, can be about 3 μm or less.

FIG. 2 shows the bandgap energies between the valence and conductionband edges for a III-V alloy system including the materials comprisingthe layers of photodetector 10 in the embodiments of FIGS. 1A and 1B.Such a material system is described in Tiwari and Frank, Appl. Phys.Lett., Vol. 60, No. 5 (1992), the entire contents of which areincorporated herein by reference.

FIG. 3 more specifically shows the bandgaps between the valence andconduction band edges for the materials comprising the substrate 12(GaSb), the photo-absorbing layers 20 (InAsSb), and thestrain-compensating layers 22 (GaAs). The data and interpolation schemefor the material system of FIG. 3 is given in Vurgaftman, Meyer andRam-Mohan, J. Appl. Phys. 89, 5815 (2001), the entire contents of whichare incorporated herein by reference. The temperature is taken as 300° Kfor these calculations. In FIG. 3, the bandgap energy between thevalence and conduction band edges of the InAsSb photo-absorbing layers20 is reduced as the Sb content of the InAsSb alloy increases, andtherefore the photo-absorbing layer exhibits a longer absorption cutoffwavelength when the Sb content in the InAsSb alloy is greater. In theembodiments of FIGS. 1A and 1B, the composition of the InAsSbphoto-absorbing layers 20 which allows for the absorption of wavelengthsgreater than 4.5 μm exhibits a lattice constant greater than that of theGaSb substrate 12. Therefore, for the photodetector 10 to exhibit thedesired extended cutoff wavelength, the InAsSb photo-absorbing layers 20of the embodiments of FIGS. 1A and 1B are grown in a lattice-mismatchedcondition with the substrate 12 such that the photo-absorbing layers 20are compressively-strained relative to the substrate 12.

The photo-absorbing layers 20 must be grown to have sufficient aggregatethickness in order for the photodetector 10 to absorb infrared light 50up to the desired extended cutoff wavelength while achieving therequired quantum efficiency. However, as the photo-absorbing layers 20are grown without compensating for the strain caused by the latticemismatch with the substrate, once a critical thickness is reached theaccumulated strain is released by generation of lattice dislocations. Ifallowed to occur these dislocations result in a decreased quantumefficiency and a higher thermal dark current, decreasing thesignal-to-noise ratio for the absorbed infrared light 50. In order tocompensate for the strain caused by the photo-absorbing layers 20 beinglattice-mismatched to the substrate 12 and therefore avoid generatingdislocations due to excess strain, the photo-absorbing region 14 isepitaxially grown on the substrate 12 by alternatingly growingstrain-compensating layers 22 interspersed between the photo-absorbinglayers 20.

In the embodiments of FIGS. 1A and 1B, the strain-compensating layers 22are comprised of GaAs, which has a lattice constant of approximately5.65 Angstroms (Å), less than the lattice constant of 6.09 Å for theGaSb substrate 12, and are therefore tensile-strained. The aggregatetensile strain of the plurality of strain-compensating layers 22substantially compensates for the aggregate compression strain of theplurality of photo-absorbing layers 20 such that dislocations do notoccur in the photo-absorbing layers 20. In certain other embodiments ofthe present invention to be described in detail hereinafter, materialsfor the substrate 12, the photo-absorbing layers 20, and thestrain-compensating layers 22 may be selected such that thephoto-absorbing layers 20 are tensile-strained relative to the substrate12 and the strain-compensating layers 22 are compression-strained;however, the net strain-balancing effect of the alternating layerswithin the photo-absorbing regions of these embodiments is substantiallythe same as that in the embodiments of FIGS. 1A and 1B.

According to the exemplary embodiments of FIGS. 1A and 1B, thephoto-absorbing region 14 can have an aggregate thickness from 1 to 10microns (μm). The InAsSb photo-absorbing layers 20 can each have athickness of, for example, about 10 nanometers (nm). The thickness ofeach of the GaAs strain-compensating layers 22 can range from 0.1-1.0nm. In general, the thickness of each of the photo-absorbing layers 20can be up to about 100 nm; and the thickness of each of thestrain-compensating layers 22 can be up to about 50 nm, the precisethickness depending on the semiconductor material used to form therespective layers. Generally, anywhere from 10 to 2000 pairs of InAsSbphoto-absorbing layers 20 and GaAs strain-compensating layers 22 maycomprise the photo-absorbing region 14.

The exact thickness of the GaAs strain-compensating layers 22 generallydepends upon an amount of tensile strain needed in thestrain-compensating layers 22 to compensate for an amount of compressivestrain in the InAsSb layers 20, such that a predetermined cutoffwavelength for the photodetector 10 is achieved. Additionally, thethickness of the strain-compensating layers 22 is preferably selectedsuch that the strain-compensating layers 22 are effectively transparentto quantum wavefunctions, enabling the collection of photogeneratedminority carriers and the attainment of a photodetector with a highercutoff frequency at the quantum efficiency required by the systemapplication, where a higher signal to noise is generally preferable.Quantum mechanically transparent thus means that the thicknesses of thestrain-compensation layers 22 little to no effect on the transmission ofelectrons there-through. The thickness of the strain-compensation layers22, while sufficient to offset the lattice mismatch strain of theabsorber 20, are too thin to alter the electron band structure of theabsorption material 20 and are not thick enough to have sufficientquantum efficiency.

while sufficient to offset the absorber strain due to lattice mismatch,are thin enough to not sufficiently alter the spectral characteristicsof the absorption material 20, such that the absorption material 20exhibits sufficient quantum efficiency.

In the exemplary embodiments of FIGS. 1A and 1B, and other embodimentsto be described hereinafter, the semiconductor layers which form thephotodetector 10 can be epitaxially grown on a semiconductor substrate12 by molecular beam epitaxy (MBE), although other known methods forgrowing the layers may be used without departing from the scope of thepresent invention. The semiconductor substrate 12 may be comprised of,for example, gallium antimonide (GaSb) or indium arsenide (InAs) and maybe undoped or doped. The InAsSb photo-absorbing layers 20 and the GaAsstrain-compensating layers 22 may grown as undoped layers (but which mayhave a non-intentional residual n-type doping), or the InAsSbphoto-absorbing layers 20 and the GaAs strain-compensating layers 22 maybe grown as n-type doped layers, although various doping concentrationsand p-type dopings may be used without departing from the scope of thepresent invention as long as the photo-absorbing and strain-compensatinglayers are substantially strain-balanced such that the photodetectorexhibits the extended cutoff wavelength and quantum efficiencycharacteristics as described herein.

The barrier layer 16 can be epitaxially grown above the photo-absorbingregion 14 and can be comprised of aluminum arsenide antimonide(AlAs_(x)Sb_(1-x)) having a composition with x generally being in therange 0≦x≦0.1. Alternately, the barrier layer can comprise aluminumgallium arsenide antimonide (AlGaAsSb) having a compositionAl_(y)Ga_(1-y)As_(x)Sb_(1-x) with 0.5≦y<1.0 and with 0≦x≦0.1. Thebarrier layer 14 can be undoped, characteristic of a barrier layer in amajority carrier filter photodetector such as disclosed by Maimon. Thebarrier layer thickness can be approximately 100 nm, for example,although other thicknesses may be considered as long as the reduced darkcurrent properties of a majority carrier filter photodetector arerealized.

Ideally, the barrier layer 16 should have a conduction band energy levelwhich is sufficiently high compared to the conduction band energy levelof the contact layer 18 to limit a thermal excitation of the majoritycarriers from the contact layer 18 over the barrier layer 16 at anoperating temperature of the photodetector 10, and should besufficiently thick to limit a tunneling of majority carriers (i.e.electrons) through the barrier layer 16. Additionally, the semiconductoralloy composition of the barrier layer 16 should be selected to providea valence band energy level that is substantially equal to the valenceband energy level of the InAsSb photo-absorbing layers 20. This allowsminority carriers (i.e. holes), which are photogenerated in thephoto-absorbing region 14 as incident infrared light 50 is absorbed, toflow with minimal bias voltage across the barrier layer 16 to thecontact layer 18 and thereby contribute to the electrical output signalof the photodetector 10. Thus, as the amount of Sb content in the InAsSbphoto-absorbing layers 20 is increased to shift the cutoff wavelength ofthe photodetector 10 to a longer wavelength in the range of 4.5 μm ormore, the semiconductor alloy composition and strain of the barrierlayer 16 can be adjusted to maintain a substantially zero offset betweenthe valence band energy levels of the barrier layer 16 and thephoto-absorbing layers 20.

In the embodiments of FIGS. 1A and 1B and the other embodiments of thepresent invention to be discussed hereinafter, the barrier layer 16 canalso act as a passivation layer to suppress surface currents due to themajority carriers, as disclosed by Maimon in U.S. Pat. No. 7,687,871. Bysuppressing surface currents and blocking a flow of the majoritycarriers from the contact layer 18 into the photo-absorbing region 14,the barrier layer 16 substantially reduces a dark current in anelectrical output signal of the photodetector 10.

In the exemplary embodiments of FIGS. 1A and 1B, the contact layer 18,which is grown above the barrier layer 18, can comprise InAs, GaSb, orInAsSb. A first-grown portion of the contact layer 18 can be undoped(i.e. not intentionally doped) with the remainder of the contact layer18 being n-type doped (e.g. with silicon) during epitaxial growth. Otherdopings such as p-type doping, as well as various doping concentrationsmay be used as long as the preferred reduced dark currentcharacteristics of a majority carrier filter photodetector are realized.An overall thickness of the contact layer 18 can be, for example, 100nm. When the InAsSb photo-absorbing layers 20 and the contact layer 18are both n-type doped and the barrier (B) layer 16 is undoped, thephotodetector 10 in FIGS. 1A and 1B can be referred to as an nBnphotodetector.

FIG. 4 shows an exemplary embodiment of the present invention whereinthe photodetector 10 has a majority carrier filter structure. FIG. 4shows the conduction and valence energy bands of the photo-absorbinglayers 20 and the strain-compensating layers 22, and in particular,depicts the increased bandgap energy of the strain-compensating layers22 with respect to the bandgap energy of the photo-absorbing layers 20.

FIG. 5 shows a more specific example of the band edge energies versusgrowth direction for the exemplary embodiment with InAsSbphoto-absorbing layers and GaAs strain-compensating layers. The GaAslayers are thin potential barriers for electron transport in the growthdirection. Because both materials are strained relative to the GaSbsubstrate, the valence band splits into heavy and light hole bands. Thisresults in the GaAs layers presenting potential barriers for heavy holetransport, while for light holes the GaAs layer is a potential wellrelative to the absorber band edge. Since a bandgap energy of thestrain-compensating layers 22 is larger than that of the InAsSblight-absorbing layers 20, the carriers (i.e. electrons and holes) thatare photogenerated in the light-absorbing region 14 in response toabsorbed infrared light 50 must be conducted across thestrain-compensating layers 22 via quantum tunneling and/or thermalexcitation to generate an electrical output signal for the photodetector10.

In the embodiments of FIGS. 1A and 1B, a top electrode 24 is provided onthe contact layer 18. The top electrode 24 can be formed of anyarbitrary shape (e.g. square, polygonal, circular, etc.) and can becomprised of materials known in the art for use with the particularsemiconductor materials described herein. As an example, the topelectrode 24 can comprise Ti/Ni formed by depositing 300 Å of Titanium(Ti) followed by 1000 Å of Nickel (Ni) or Ge/Au/Ni/Au metallizationformed by depositing 26 nm of germanium (Ge), 54 nm of gold, 15 nm ofnickel (Ni) and 200 nm of gold in that order.

In the example of FIG. 1A, a bottom electrode 26 can be deposited over aportion of the bottom surface of the GaSb substrate 12 such that it isconnected electrically to the photo-absorbing region 14 through thesubstrate 12. The bottom electrode 26 can be comprised of materials asdescribed previously with respect to the top electrode 24. In abackside-illuminated arrangement as is known in the art, infrared light50 impinging on the photodetector can be transmitted through the GaSbsubstrate 12 to the photo-absorbing region 14 through an opening 34formed through the bottom electrode 26, as shown in FIG. 1A.Alternately, in a topside-illuminated arrangement (not shown) as isknown in the art, the top electrode 24 can be formed in an annular shapesuch that infrared light 50 is absorbed into the photo-absorbing region14 from a top side of the substrate 12. In a preferred embodiment, shownin FIG. 1B, both electrodes 24 and 26 are formed on the top side of thephotodetector. In this embodiment, portions of the contact layer 18 andthe barrier layer 16 are sufficiently laterally removed from the etchdown to the absorber top surface 30 such that the detector dark currentis not increased.

In the exemplary embodiments of FIGS. 1A and 1B, the contact layer 18 isselectively etched down to the barrier layer 16 to define a lateralextent of the photodetector 10. The selective etching can be performed,for example, using a wet etchant comprising a solution of citric acidand hydrogen peroxide. The selective etchant does not substantially etchthe AlAsSb or AlGaAsSb used for the barrier layer 16, such that theetching process stops at the barrier layer 16. In this manner, theetched contact layer 18 forms a mesa on the barrier layer 16 such thatthe barrier layer laterally extends beyond the contact layer 18, therebypassivating the photodetector 10 during operation by preventing majoritycarriers from flowing to exposed surfaces of said barrier layer 16.

In the preferred embodiment of FIG. 1B, a via 34 can be etched in thebarrier layer 16 using a non-selective etch that is timed to etch to thephoto-absorbing layer 14. The bottom electrode 26 is then deposited intothe via 34. Other etching methods and placement locations for the bottomelectrode 26 may also be used in addition to those described hereinwithout deviating from the inventive concepts described herein.

In another embodiment of the present invention as shown in FIG. 14, aplurality of photodetectors 10 are formed on a common semiconductorsubstrate 12 so as to form a focal plane array 100 of photodetectors 10.FIG. 15A shows a top schematic view of the focal plane array 100 inwhich the contact layer 16 is shown as selectively etched down to thebarrier layer 16 to form individual sections delineated by etchedtrenches 32, which define a lateral extent of each of the photodetectors10. In this embodiment, the barrier layer extends past the individualsections of the contact layer in a lateral direction across thephotodetector, and is monolithically provided for each of the individualphotodetectors 10, thereby passivating the photodetectors 10 duringoperation by blocking the flow of majority carriers to exposed surfacesof said barrier layer 16.

In the preferred embodiment of FIG. 15B, the construction follows thearray as in FIG. 15A but a via 34 is etched as in FIG. 1B around theperiphery of the array. With both contacts 24 and 26 disposed on the topside of the photodetector, indium bumps can be deposited and the arraycan be hybridized to a readout integrated circuit (ROIC) in the mannerknown to those skilled in the art. With the backside clear for thinningand antireflection coatings, the quantum efficiency of the photodetectoris maximized while the processing is simplified. In some cases the wafersubstrate may be partially or completely removed prior to theapplication of an antireflection coating.

The photodetector 10 formed according to the exemplary embodiments ofFIGS. 1A and 1B can be operated with a small reverse-bias appliedbetween the electrodes 24 and 26. When infrared light 50 is absorbed,electrons and holes are photogenerated within the photo-absorbing region14 and flow to the bottom electrode 26 under an electric field generatedby the applied reverse-bias voltage, where they are collected toconstitute part of the output signal from the photodetector 10. Thephotogenerated holes move in an opposite direction and are collected inthe contact layer 18 to further contribute to the electrical outputsignal of the photodetector 10. Because of the quantum-mechanicaltransparency of the strain-compensating layers 22, the photogeneratedelectrons and holes are conducted through the strain-compensating layers22 by tunneling and/or thermal excitation.

According to aspects of the majority carrier filter photodetector of thepresent invention, the barrier layer 16 exhibits a valence band energylevel substantially the same as that of the adjacent photo-absorbinglayer 20 such that conduction of the photogenerated holes across thebarrier layer 16 to the contact layer 18 is not impeded. On the otherhand, there exists a substantial offset in the conduction band betweenthe conduction band energy level of the barrier layer 16 and theconduction band energy level of the contact layer 18. This substantialoffset in the conduction band energy levels between the barrier layer 16and the contact layer 18 is effective to substantially limit the flow ofmajority carrier electrons across the barrier layer 16 from the contactlayer 18 to the photo-absorbing region 14. This has the beneficialeffect of a greatly reduced dark current noise level in the outputsignal from the photodetector 10. The barrier layer 16 also acts as apassivation layer for the photodetector 10, as disclosed by Maimon,effectively limiting surface currents which also contribute to the darkcurrent in conventional photodetectors.

By providing the strain-compensating layers 22 in the photo-absorbingregion 14, the InAsSb photo-absorbing layers 20 can be oppositelystrained (i.e. compressively strained) by increasing the amount ofantimony in the layers 20; and this compressive strain in thelight-absorbing layers 20 can be accommodated. Increasing the amount ofantimony in the InAsSb photo-absorbing layers 20 is advantageous toextending the cutoff to longer wavelengths for the detection of infraredlight 50 in the photodetector 10. With the layers 20 and 22 beingoppositely strained, an overall strain in the photo-absorbing region 14can be balanced and minimized when averaged over a number of the layers20 and 22 so that an effective lattice constant for the photo-absorbingregion 14 is about the same as the lattice constant of the semiconductorsubstrate 12. In this way, the photo-absorbing region 14 can besubstantially lattice-matched to the substrate 12. These properties ofthe exemplary embodiment of the present invention are described in moredetail hereinafter.

As discussed above, in order to achieve a substantially strain-balancedstate for the photo-absorbing region 14, the photo-absorbing region 14must have an overall effective lattice constant equal (or very close to)the lattice constant of the substrate. For a photo-absorbing regioncomposed of photo-absorbing layers of material “a” andstrain-compensating layers of material “b,” the effective latticeconstant of the photo-absorbing region 14 is given bya _(eff)=(a _(a) t _(a) +a _(b) t _(b))/(t _(a) +t _(b))  (1)where a_(eff), a_(a), and a_(b) are the lattice constants for thephoto-absorbing region 14, the photo-absorbing layers 20, and thestrain-compensating layers 22, respectively; and t_(a) and t_(b) are thethicknesses of the individual photo-absorbing and strain-compensatinglayers, respectively. Solving this equation for the ratio of thethicknesses results in

$\begin{matrix}{\frac{t_{a}}{t_{b}} = {\frac{a_{b} - a_{eff}}{a_{eff} - a_{a}}.}} & (2)\end{matrix}$

To achieve ideal strain-balancing, the effective lattice constant of thephoto-absorbing region 14 is set equal to that of the substrate 12. Thecondition for the ratio of thicknesses becomes

$\begin{matrix}{\frac{t_{a}}{t_{b}} = {\frac{a_{b} - a_{substrate}}{a_{substrate} - a_{a}}.}} & (3)\end{matrix}$

Therefore, the optimal thickness ratio between the photo-absorbinglayers and the strain-compensating layers which achievesstrain-balancing for the photo-absorbing region can be determined fromthe lattice constants for a particular substrate semiconductor material,strain-compensating material, and photo-absorbing material composition.Furthermore, the choice of the photo-absorbing material compositiondepends on the desired absorption cutoff wavelength for thephotodetector. As previously discussed with respect to the embodimentsof FIGS. 1A and 1B, for example, an increase in the Sb content of anInAsSb absorber reduces the bandgap energy for the absorber and extendsthe cutoff frequency to a particular value. The lattice constant ofInAs(1−x)Sb(x) absorber material is calculated given the materialtemperature and composition. The composition is denoted by the antimonymole fraction, x. Assuming GaAs strain-compensating layers and a GaSbsubstrate, the ratio of layer thicknesses is calculated using expression(3) above.

Thus, according to the present invention, the III-V semiconductor alloycomposition of the InAsSb light-absorbing layers 20 can beInAs_(x)Sb_(1-x) with 0≦X≦0.9 to provide a longer cutoff wavelength forthe photodetector 10 which is at a predetermined wavelength in the rangeof 4.5 μm or more at an operating temperature of, for example, 160° K orless.

FIG. 7 shows an exemplary method of designing an extended wavelengthphotodetector with particular strain-compensation characteristics inorder to achieve a particular cutoff wavelength while maintainingreasonable quantum efficiency. In a particular example, in order tocompensate for the strain of the InAsSb absorber layers 20 enough toeffect a particular desired cutoff wavelength, a period for thestrain-compensated pulses can be determined according to the method, asdiscussed below.

In step S010, a particular desired cutoff wavelength is selected for thephotodetector. In step S020, based on the desired cutoff wavelength, thecorresponding Sb mole fraction for InAsSb that absorbs infrared light upto the desired cutoff wavelength is determined, for example, accordingto the relationship between the Sb mole fraction and the cutoffwavelength for InAs(1−x)Sb(x). This relationship is depicted in FIG. 8.Next, in step S030, the ratio of layer thickness that achieves aneffective lattice constant for the photo-absorbing region 14 that issubstantially similar to the lattice constant of the substrate material(and thereby achieves strain-balancing of the photo-absorbing layer 14)is determined according to the lattice constant of InAsSb having thedetermined Sb mole fraction, the lattice constant of the GaSb substrate,and the lattice constant of the GaAs strain-compensating layers based onthe expression (3) described above. The relationship between the Sb molecontent and the thickness ratio is shown in FIG. 9 for the particularmaterials of the embodiments of FIGS. 1A and 1B. Then, in step S040, aperiodic interval for the GaAs strain-compensating layers is selectedsuch that the strain-compensating layer thickness remains effectivelyquantum mechanically transparent without introducing significantconfinement energy that offsets the wavelength extension from the changein photo-absorber composition and thereby do not significantly affectthe spectral properties of the overall photo-absorbing region. Finally,in step S050, the photo-absorbing layers and strain-compensation layersare alternatingly grown on the substrate at the determined periodicinterval and at a quantum-mechanically transparent thickness.

A particular example of the method of FIG. 7 will be more specificallydescribed herein. First, a desired cutoff wavelength of 5 μm is chosen.Next, from FIG. 8, the Sb content x corresponding to a cutoff wavelengthof 5 μm is approximately 0.14. From FIG. 9, x=0.14 corresponds to athickness ratio of approximately 0.05. Therefore, for GaAs straincompensating layers with a thickness of 5 Å, for example, the thicknessfor the InAs(0.86)Sb(0.14) photo-absorbing layers corresponds to 100 Å,although other thicknesses corresponding to the ratio of 0.05. Aphoto-absorbing region grown with these characteristics exhibits anabsorption cutoff wavelength very close to that of the bulk ternaryInAs(0.86)Sb(0.14) material, with greater quantum efficiency as the GaAsstrain-compensating layers are selected to be thinner. FIG. 20 depictsmeasured data confirming that the cutoff wavelength is extended to 5 μm.Although the materials for the preferred embodiments of FIGS. 1A and 1Bare used in this example, a skilled artisan will recognize that thismethod is not limited to these materials and is readily applicable toother comparable materials.

Further explanation of the desirability of selecting thestrain-compensating layer thickness is provided herein. The analysis ofcarrier transport through potential barriers is well known, for example“Quantum Mechanics,” Cohen-Tannoudji, Hermann, Paris, France 1977, pp67-78, the entire contents of which are incorporated herein byreference. Referring to FIG. 10, the quantum mechanical solution totransmission of carriers through potential barriers and wells is givenby

$\begin{matrix}{{T(E)} = \frac{\left( {2\; k\;\kappa} \right)^{2}}{{\left( {k^{2} + \kappa^{2}} \right)^{2}{\sinh^{2}\left( {\kappa\; a} \right)}} + \left( {2\; k\;\kappa} \right)^{2}}} & (4)\end{matrix}$withκ=√{square root over (2m(V ₀ E))}/η  (5)andk=√{square root over (2m(E))}/η  (6)where m is the mass of the particle, a is the thickness of the barrieror well, V_(o) is the height of the potential barrier or depth of thewell relative to the absorber band edge, and E is the energy of theparticle relative to the absorber band edge. In the case of an electronin a heterostructure, the particle mass is known as the effective mass.See, for example, “Principles of the Theory of Solids,” J. M. Ziman,Cambridge University Press, Cambridge, England, 1972, the entirecontents of which are incorporated herein by reference. The solutions tothe crystal potentials result in different effective masses near theconduction (electrons) and valence (holes) band edges.

Results of the transmission calculations are shown in FIG. 11 forelectrons (top), light holes (middle) and heavy holes (bottom). Theparticle energies are chosen to be 0.5*kT, kT, 2*kT and 3*kT above theternary band edges. The temperature is taken as 200° K. Thestrain-compensating layers are GaAs with thicknesses from 1-10 Å. Foreach GaAs layer thickness in the plots, the InAsSb composition is chosento strain-balance the photo-absorbing region relative to a GaSbsubstrate. As expected, the transmission decreases with increasing GaAsthickness. Transmissions of ˜50% for GaAs layers of 10 Å or less arequite acceptable. While the heavy hole transmission drops rapidly, theband mixing at the heterointerfaces ensure that the hole carriertransport will be sufficient to ensure detector operation.

A more complete theoretical basis for digital alloy design can be foundin mini-band theory. Assuming an infinite lattice, Kronig-Penneysolutions are used to calculate the modes and mini-bands. The mini-bandsare plotted in FIG. 12, along with the band edges along the growthdirection. From FIG. 12, the energy gap (and hence the absorption cutoffwavelength) is nearly the same for the photo-absorbing region (includingthe strain-compensating layers) as for a bulk ternary absorber materialwithout any strain-compensating layers. Thus, thin layers of thestrain-compensating material do not substantially alter the absorptioncharacteristics of the ternary material.

The two hole band maxima are essentially at the same energy value,therefore the absorbed light will generate both light and heavy holesavailable for transport, unlike most super-lattices where theconfinement energy places the heavy holes closer to the band edge thanthe light holes. This energy overlap of heavy and light holes allows forscatter transfer and improves the overall diffusion transport ofphoto-generated carriers in a majority-carrier filter photodetector.

An important consideration for the transport of holes is the band edgealignment between the hole mini-bands and the barrier valence band edge,as shown in FIG. 13. According to aspects of the present invention, bandedge alignment can be achieved by proper choice of the barrier material,and by choosing the ratio of widths for the alternating layers of thephoto-absorbing region 14. When the thickness of the strain-compensatinglayers 22 is kept below a critical thickness such that they aresubstantially quantum-mechanically transparent, both strained andunstrained photo-absorber regions may be used to modify the valence bandedge of the barrier layer. This means that the composition of thecontact layer may also be formed to comprise alternating photo-absorbinglayers and strain-compensating layers similarly to that of thephoto-absorbing region 14 of FIG. 1.

Accordingly, FIG. 18 shows an alternative embodiment of the presentinvention in which, in addition to providing a strain-balanced structurefor photo-absorbing region 14, contact layer region 58 is also formed tohave a strain-balanced structure. Thus, it can be appreciated that, forany mention of a “contact layer” in any of the foregoing and ensuingdescription within the instant disclosure, one of ordinary skill in theart will understand that such a contact layer may be formed of astrain-balanced construction, e.g., as shown in FIG. 18, while remaininga part of the inventive concept as described herein. Additionally, sucha strain-balanced contact layer may alternatively be referred to as anadditional photo-absorbing region formed above the barrier layer,without departing from the scope of the present invention. Furtherembodiments of the present invention with respect to a strain-balancedcontact layer are described in greater detail hereinafter with respectto FIG. 18.

FIG. 18 shows a schematic cross-section view of a strain-balancedtwo-color photodetector according to an embodiment of the presentinvention. In FIG. 18, photo-absorbing region 14 is located on substrate12 and is comprised of alternating layers of a plurality of firstphoto-absorbing layers 20 and a first plurality of strain-compensatinglayers 22. A barrier layer 16 is located above the photo-absorbingregion 14. A contact region 58 is disposed on the barrier layer 16 andcomprises alternating layers of a plurality of second photo-absorbinglayers 60 and a second plurality of strain-compensating layers 22. For aphotodetector formed to have such a construction, the contact layerregion 58 may be comprised of alternating layers of, e.g., InAsSbphoto-absorbing layers and GaAs strain-compensating layers, comparableto the photo-absorbing region 14 in the example of FIG. 1, and mayexhibit conduction and valence bandgap energies as depicted in FIG. 13.As discussed further below, and as will be appreciated by one ofordinary skill in the art, alternative material systems exhibitingdifferent tailorable cutoff wavelengths may be used.

FIG. 16 shows an example of a material system corresponding to analternative embodiment of the present invention in which the substrateis comprised of InAs instead of the GaSb as in the embodiments of FIGS.1A and 1B. This enables the InAsSb ternary absorber to achieveabsorption cutoff from that of InAs through the maximum achievable fromInAsSb, all on the same substrate and with the same choice of GaAsstrain-compensating material. FIG. 16 shows the bandgap energies betweenthe valence and conduction band edges for an InAs substrate, InAsSbphoto-absorbing layers, and GaAs strain-compensating layers, wherein theInAsSb absorber layers are compressively strained as the antimonycontent is increased.

FIG. 17 shows the bandgap energies between the valence and conductionband edges of a material system corresponding to a further alternativeembodiment of the present invention. In the embodiment of FIG. 17, thephoto-absorbing region will exhibit a cutoff wavelength increasinglysmaller than 4.2 μm as the antimony content of the InAs(1−x)Sb(x)absorber material is decreased from the lattice-matched value x≈0.09.Once the antimony content of the absorber material is zero, the additionof gallium into the InAs material will further lower the latticeconstant below that of the GaSb substrate, resulting in a InGaAsabsorber material that is tensile-strained. To balance thistensile-strained absorber, thin layers of InSb strain-compensatinglayers can be interspersed alternatingly with the InGaAs photo-absorbinglayers in the same manner as described with respect to the embodimentsof FIGS. 1A and 1B, with the exception that the strain components arereversed from that of FIG. 1. Owing to these features, a cutoffwavelength for a strain-balanced photo-absorbing region may be tailoredto be a particular value less than that of 4.2 μm, while thephoto-absorbing region is in a substantially lattice-mismatchedcondition with the substrate.

Based on the foregoing, and according to a further embodiment of thepresent invention, a photodetector may be constructed with twophoto-absorbing regions on either side of the barrier layer, eachpotentially exhibiting a different cutoff wavelength. Thus, aphotodetector constructed such as shown in FIG. 18 may be tailored todetect two specific and desired colors of infrared light whilemaintaining a strain-balanced construction with a reduced dark current.FIG. 19 shows an example of band edge alignment between the holemini-bands and the barrier valence band edge of a strain-balancedtwo-color photodetector exhibiting two different long cutoffwavelengths, according to an alternative embodiment of FIG. 18.

FIG. 20 shows a photodetector with a strain-compensation graded bandgapadapted from a graded bandgap design as disclosed by Scott et al. inU.S. Pat. Pub. No. 2008/0111152 A1. In FIG. 20, the bandgap of thephoto-absorbing layer 20 decreases as the distance from the barrier 16decreases. The decrease in the bandgap of photo-absorbing layer 20 isachieved by increasing the Sb content x in the InAs(1−x)Sb(x)photo-absorbing layer 20 in the direction of the barrier. The chemicalpotential caused by the increasing Sb content creates a quasi-field thatpushes the minority carriers into the barrier 16 for collection. Thisquasi-field has the beneficial quality of reducing the effect of lateraldiffusion in the planar absorber that can result in crosstalk ofphotogenerated carriers between adjacent pixels.

Since the Sb content of the photo-absorbing layer 20 increases in thedirection of the barrier 16, the lattice constant of the photo-absorbingmaterial also increases in the direction of the barrier 16, creating alattice-mismatched condition during growth of the photo-absorbing layer20 on the substrate. In order to strain-balance the photo-absorbinglayer 20 having a graded bandgap, strain-compensating layers 22 may beinterspersed within the photo-absorbing layer 20 at a graduallyincreasing periodicity in the direction of the barrier layer 16. In analternative embodiment of the present invention, strain-balancing of thephoto-absorbing layer 20 may instead be achieved by interspersingincreasingly thicker strain-compensating layers in the direction of thebarrier layer 16. Alternatively, strain-balancing of the photo-absorbinglayer 20 may be achieved by a combination of a decreasing periodicityand an increasing thickness for the strain-compensating layers.

Only exemplary embodiments of the present invention are shown anddescribed in the present disclosure. It is to be understood that thepresent invention is capable of use in various other combinations andenvironments and is capable of changes or modifications with the scopeof the inventive concept as expressed herein. Such variations are not tobe regarded as departure from the spirit and scope of the invention, andall such modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims:

What is claimed is:
 1. A strain-balanced photodetector with an extendedwavelength, comprising: a first layer grown on a semiconductorsubstrate; a barrier layer located above the first layer; and a secondlayer located above the barrier layer, wherein the first layer comprisesa plurality of photo-absorbing layers; and a plurality ofstrain-compensating layers interspersed between the plurality ofphoto-absorbing layers, wherein the photo-absorbing layers are grownsubstantially lattice-mismatched to the substrate, and thestrain-compensating layers are interspersed between the photo-absorbinglayers so as to substantially compensate for a mechanical strain of thephoto-absorbing layers caused by the lattice-mismatched condition; andwherein each of the plurality of strain-compensating layers has athickness such that the strain-compensating layers are substantiallyquantum-mechanically transparent.
 2. The photodetector of claim 1,wherein the photo-absorbing layers are stacked in an alternatingarrangement with the strain-compensating layers, with one of thestrain-compensating layers being located between each pair of thephoto-absorbing layers.
 3. The photodetector of claim 1, wherein theplurality of photo-absorbing layers are comprised of indium arsenideantimonide (InAsSb) or indium gallium arsenide (InGaAs); and theplurality of strain-compensating layers are comprised of galliumarsenide (GaAs) or indium antimonide (InSb).
 4. The photodetector ofclaim 3, wherein the substrate is comprised of gallium antimonide (GaSb)or indium arsenide (InAs).
 5. The photodetector of claim 1, wherein thefirst layer has an aggregate thickness sufficient to exhibitphoto-absorption with reasonable quantum efficiency while exhibiting anextended long cutoff frequency beyond that of a photodetector with afirst layer that is grown lattice-matched to a substrate.
 6. Thephotodetector of claim 1, wherein a portion of the second layer isetched down to the barrier layer to define a lateral extent of thephotodetector.
 7. The photodetector of claim 1, wherein the second layercomprises individual sections which are delineated from each other in adirection across the photodetector, each section corresponding to anindividual detector element, wherein said barrier layer extends past theindividual sections of the second layer in the direction across thephotodetector, and is monolithically provided for each of the individualdetector elements, thereby passivating the photodetector duringoperation by blocking the flow of majority carriers to exposed surfacesof said barrier layer.
 8. The photodetector of claim 1, wherein thesecond layer forms a mesa on the barrier layer such that the barrierlayer laterally extends beyond the mesa thereby passivating thephotodetector during operation by preventing majority carriers fromflowing to exposed surfaces of said barrier layer.
 9. The photodetectorof claim 1, wherein the first and second layers have the same majoritycarrier type such that the photodetector has no substantial depletionlayer.
 10. The photodetector of claim 1, wherein the barrier layer has asemiconductor alloy composition AlAs_(x)Sb_(1-x) with x being selectedto provide a valence band energy for the barrier layer which issubstantially equal to the valence band energy of the plurality ofphoto-absorbing layers.
 11. The photodetector of claim 1, wherein thebarrier layer comprises aluminum gallium arsenide antimonide (AlGaAsSb).12. The photodetector of claim 11, wherein the AlGaAsSb barrier layerhas a semiconductor alloy composition AlGa_(1-y)As_(x)Sb_(1-x) with0.5≦y<1.0 and with 0≦x≦0.1 being selected to provide a valence bandenergy for the barrier layer which is substantially equal to the valenceband energy of the plurality of photo-absorbing layers.
 13. Thephotodetector of claim 1, wherein the photo-absorbing layers compriseInAs_(x)Sb_(1-x) with 0≦x≦0.9.
 14. The photodetector of claim 13,wherein each photo-absorbing layer has a layer thickness of about 100nanometers or less.
 15. The photodetector of claim 13, wherein eachstrain-compensating layer has a layer thickness of 5 nanometers or less.16. The photodetector of claim 1, wherein a long cutoff wavelength ofthe photodetector is in a range of 4.5 to 10 μm at a temperature of 200°K or less.
 17. The photodetector of claim 1, wherein the plurality ofphoto-absorbing layers comprise 10 to 2000 photo-absorbing layers. 18.The photodetector of claim 1, wherein the first layer has a thickness inthe range of 1 to 10 μm.
 19. The photodetector of claim 1, wherein thelattice constant of the photo-absorbing layers is larger than thelattice constant of the substrate such that each photo-absorbing layeris compression-strained and each strain-compensating layer istensile-strained.
 20. The photodetector of claim 1, wherein the latticeconstant of the photo-absorbing layers is smaller than the latticeconstant of the substrate such that each photo-absorbing layer istensile-strained and each strain-compensating layer iscompression-strained.
 21. The photodetector of claim 1, wherein thefirst and second layers exhibit substantially identical compositions.22. The photodetector of claim 1, wherein the first and second layersare either both n-type or both p-type.
 23. The photodetector of claim 1,wherein the barrier layer comprises a substantially undoped material.24. The photodetector of claim 1, further comprising: a first electrodeelectrically connected to the first layer; and a second electrodeelectrically connected to the second layer.
 25. The photodetector ofclaim 1, where the first layer comprises a plurality of compressivelystrained photo-absorbing layers of indium arsenide antimonide (InAsSb)grown alternatingly with a plurality of strain-compensating layers ofgallium arsenide (GaAs).
 26. The photodetector of claim 25, wherein thesemiconductor substrate is comprised of gallium antimonide (GaSb). 27.The photodetector of claim 25, wherein each InAsSb photo-absorbing layercomprises InAs_(x)Sb_(1-x) with 0≦x≦0.9.
 28. The photodetector of claim25, wherein each GaAs layer has a thickness which is less than or equalto one-fifth of the thickness of an adjacent InAsSb layer.
 29. Thephotodetector of claim 25, wherein the first layer has a total thicknessin the range of 1 to 10 μm.
 30. The photodetector of claim 25, wherein along cutoff wavelength for the detection of light is in the range of 4.5to 10 μm at a temperature of 160° K or less.
 31. The photodetector ofclaim 25, wherein the first layer comprises 10 to 2000 pairs ofalternating layers of InAsSb and GaAs.
 32. The photodetector of claim25, wherein the barrier layer comprises aluminum arsenide antimonide(AlAsSb).
 33. The photodetector of claim 25, further comprising: a firstelectrode electrically connected to the first layer; and a secondelectrode electrically connected to the second layer.
 34. Thephotodetector of claim 1, wherein the first layer exhibits a firstcutoff wavelength and the second layer exhibits a second cutoffwavelength, and the first cutoff wavelength is shorter than the secondcutoff wavelength such that the photo-detector is configured fortwo-color operation.
 35. The photodetector of claim 1, wherein the firstlayer comprises an alloy of indium arsenide antimonide (InAsSb)substantially lattice-matched togallium antimonide (GaSb); and thesecond layer comprises a plurality of tensile-strainedstrain-compensating layers interspersed between a plurality ofcompressive-strained photo-absorbing layers comprised of indium galliumarsenide antimonide (InGaAsSb).
 36. The photodetector of claim 35,wherein the strain-compensating layers are comprised of gallium arsenide(GaAs).
 37. The photodetector of claim 1, wherein the first layercomprises a plurality of compressive-strained strain-compensating layerscomprised of indium arsenide antimonide (InAsSb) of InAs_(w)Sb_(1-w)with 0≦w≦0.9 interspersed between a plurality of tensile-strainedphoto-absorbing layers comprised of indium gallium arsenide antimonide(InGaAsSb) of In_(x)Ga_(1-x)As_(y)Sb_(1-y) with 0≦x≦1.0 and 0≦y≦1.0; andthe second layer comprises an alloy of indium arsenide antimonide(InAsSb) substantially lattice-matched to togallium antimonide (GaSb).38. The photodetector of claim 37, wherein the strain-compensatinglayers are comprised of indium antimonide (InSb).
 39. The photodetectorof claim 1, wherein the first layer comprises a plurality ofcompressive-strained strain-compensating layers comprised of indiumarsenide antimonide (InAsSb) of InAs_(w)Sb_(1-w) with 0≦w≦0.9interspersed between a plurality of tensile strained photo-absorbinglayers comprised of indium gallium arsenide antimonide (InGaAsSb) ofIn_(x)Ga_(1-x)As_(y)Sb_(1-y) with 0≦x≦1.0 and 0≦y≦1.0.
 40. Thephotodetector of claim 39, where the strain-compensating layers of thefirst layer are comprised of indium antimonide (InSb); and thestrain-compensating layers of the second layer are comprised of galliumarsenide (GaAs).
 41. The photodetector of claim 1, wherein the firstlayer comprises a plurality of photo-absorbing layers comprised ofindium arsenide antimonide (InAsSb) and a plurality ofstrain-compensating layers comprised of gallium arsenide (GaAs); and thesecond layer comprises a plurality of photo-absorbing layers comprisedof indium gallium antimonide (InGaSb) and a plurality ofstrain-compensating layers comprised of indium antimonide (InSb). 42.The photodetector of claim 1, wherein the substrate comprises gallimantimonide (GaSb) or indium arsenide (InAs); and the barrier layercomprises aluminum arsenide antimonide (AlAsSb) or aluminum galliumarsenide antimonide (AlGaAsSb).
 43. The photodetector of claim 1,wherein a minority carrier band edge of at least one of the first andsecond layers is graded vertically by varying the alloy composition in adirection toward the barrier layer, and wherein a plurality of varyingstrain-compensating layers are interspersed within said at least one ofthe first and second layers such that the lattice structure issubstantially prevented from dislocating.
 44. The photodetector of claim43, wherein the interval between the strain-compensating layers isgradually varied in the direction toward the barrier layer in order tobalance the varying strain within the at least one of the first andsecond layers.
 45. The photodetector of claim 44, wherein the thicknessof the strain-compensating layers is gradually varied in the directiontoward the barrier layer in order to balance the varying strain withinthe at least one of the first and second layers.
 46. The photodetectorof claim 43, wherein the thickness of the strain-compensating layers isgradually varied in the direction toward the barrier layer in order tobalance the varying strain within the at least one of the first andsecond layers.
 47. The photodetector of claim 46, wherein the intervalbetween the strain-compensating layers is gradually varied in thedirection toward the barrier layer in order to balance the varyingstrain within the at least one of the first and second layers.
 48. Afocal plane array with an extended cutoff wavelength, comprising: aplurality of photodetectors according to claim 1, arranged in atwo-dimensional matrix.
 49. A strain-balanced photodetector with anextended wavelength, comprising: a first layer grown on a semiconductorsubstrate; a barrier layer located above the first layer; and a secondlayer located above the barrier layer, wherein the first layer comprisesa plurality of photo-absorbing layers; and a plurality ofstrain-compensating layers interspersed between the plurality ofphoto-absorbing layers, wherein the photo-absorbing layers are grownsubstantially lattice-mismatched to the substrate, and thestrain-compensating layers are interspersed between the photo-absorbinglayers so as to substantially compensate for a mechanical strain of thephoto-absorbing layers caused by the lattice-mismatched condition; andwherein the plurality of strain-compensating layers are interspersedbetween the plurality of photo-absorbing layers at a periodic intervalsuch that the strain-compensating layers are substantially transparentto quantum waveforms.
 50. A strain-balanced photodetector with anextended wavelength, comprising: a first layer grown on a semiconductorsubstrate, where the first layer exhibits a valence band energy and aconducting band energy during operation of the photodetector; a barrierlayer located above the first layer, where the barrier layer has a bandenergy gap and associated conduction and valence band energies; and asecond layer located above the barrier layer, where the second layerexhibits a valence band energy and a conducting band energy duringoperation of the photodetector, wherein the relationship between thefirst and second layer valence and conduction band energies and thebarrier layer conduction and valence band energies facilitates minoritycarrier current flow while inhibiting majority carrier current flowbetween the first and second layers; and wherein the first layerincludes a plurality of photo-absorbing layers; and a plurality ofstrain-compensating layers interspersed between the plurality ofphoto-absorbing layers, and wherein the photo-absorbing layers are grownsubstantially lattice-mismatched to the substrate, and thestrain-compensating layers are interspersed between the photo-absorbinglayers so as to substantially compensate for a mechanical strain of thephoto-absorbing layers caused by the lattice-mismatched condition.
 51. Astrain-balanced photodetector with an extended wavelength, comprising: afirst layer grown on a semiconductor substrate; a barrier layer locatedabove the first layer; and a second layer located above the barrierlayer, wherein the second layer comprises a plurality of photo-absorbinglayers; and a plurality of strain-compensating layers interspersedbetween the plurality of photo-absorbing layers, wherein thephoto-absorbing layers are grown substantially lattice-mismatched to thebarrier layer, and the strain-compensating layers are interspersedbetween the photo-absorbing layers so as to substantially compensate fora mechanical strain of the photo-absorbing layers caused by thelattice-mismatched condition; and wherein each of the plurality ofstrain-compensating layers has a thickness such that thestrain-compensating layers are substantially quantum-mechanicallytransparent.
 52. The photodetector of claim 51, wherein the latticeconstant of the photo-absorbing layers is larger than the barrier layersuch that each photo-absorbing layer is compression-strained and eachstrain-compensating layer is tensile-strained.
 53. The photodetector ofclaim 51, wherein the lattice constant of the photo-absorbing layers issmaller than the barrier layer such that each photo-absorbing layer istensile-strained and each strain-compensating layer iscompression-strained.
 54. The photodetector of claim 51, wherein thephoto-absorbing layers are stacked in an alternating arrangement withthe strain-compensating layers, with one of the strain-compensatinglayers being located between each pair of the photo-absorbing layers.55. The photodetector of claim 51, wherein the plurality ofphoto-absorbing layers are comprised of indium arsenide antimonide(InAsSb) or indium gallium arsenide (InGaAs); and the plurality ofstrain-compensating layers are comprised of gallium arsenide (GaAs) orindium antimonide (InSb).
 56. The photodetector of claim 55, wherein thesubstrate is comprised of gallium antimonide (GaSb) or indium arsenide(InAs).
 57. The photodetector of claim 51, wherein the first layer hasan aggregate thickness sufficient to exhibit photo-absorption withreasonable quantum efficiency while exhibiting an extended long cutofffrequency beyond that of a photodetector with a first layer that isgrown lattice-matched to a substrate.
 58. The photodetector of claim 51,wherein a portion of the second layer is etched down to the barrierlayer to define a lateral extent of the photodetector.
 59. Thephotodetector of claim 51, wherein the second layer comprises individualsections which are separate from each other in a direction across thephotodetector, each section corresponding to an individual detectorelement, wherein said barrier layer extends past the individual sectionsof the second layer in the direction across the photodetector, and ismonolithically provided for each of the individual detector elements,thereby passivating the photodetector during operation by blocking theflow of majority carriers to exposed surfaces of said barrier layer. 60.The photodetector of claim 51, wherein the second layer forms a mesa onthe barrier layer such that the barrier layer laterally extends beyondthe mesa thereby passivating the photodetector during operation bypreventing majority carriers from flowing to exposed surfaces of saidbarrier layer.
 61. The photodetector of claim 51, wherein the first andsecond layers have the same majority carrier type such that thephotodetector has no substantial depletion layer.
 62. The photodetectorof claim 51, wherein the barrier layer has a semiconductor alloycomposition AlAs_(x)Sb_(1-x) with x being selected to provide a valenceband energy for the barrier layer which is substantially equal to thevalence band energy of the plurality of photo-absorbing layers.
 63. Thephotodetector of claim 51, wherein the barrier layer comprises aluminumgallium arsenide antimonide (AlGaAsSb).
 64. The photodetector of claim51, wherein the first and second layers exhibit substantially identicalcompositions.
 65. The photodetector of claim 51, wherein the first andsecond layers are either both n-type or both p-type.
 66. Thephotodetector of claim 51, wherein the barrier layer comprises asubstantially undoped material.
 67. The photodetector of claim 51,further comprising: a first electrode electrically connected to thefirst layer; and a second electrode electrically connected to the secondlayer.
 68. The photodetector of claim 51, wherein a minority carrierband edge of at least one of the first and second layers is gradedvertically by varying the alloy composition in a direction toward thebarrier layer, and wherein a plurality of varying strain-compensatinglayers are interspersed within said at least one of the first and secondlayers such that the lattice structure is substantially prevented fromdislocating.
 69. A strain-balanced photodetector with an extendedwavelength, comprising: a first layer grown on a semiconductorsubstrate; a barrier layer located above the first layer; and a secondlayer located above the barrier layer, the second layer including aplurality of photo-absorbing layers; and a plurality ofstrain-compensating layers interspersed between the plurality ofphoto-absorbing layers, wherein the photo-absorbing layers are grownsubstantially lattice-mismatched to the barrier layer, and thestrain-compensating layers are interspersed between the photo-absorbinglayers so as to substantially compensate for a mechanical strain of thephoto-absorbing layers caused by the lattice-mismatched condition; andwherein the plurality of strain-compensating layers are interspersedbetween the plurality of photo-absorbing layers at a periodic intervalsuch that the strain-compensating layers are substantially transparentto quantum waveforms.
 70. A strain-balanced photodetector with anextended wavelength, comprising: a first layer grown on a semiconductorsubstrate, where the first layer exhibits a valence band energy and aconducting band energy during operation of the photodetector; a barrierlayer located above the first layer, where the barrier layer has a bandenergy gap and associated conduction and valence band energies; and asecond layer located above the barrier layer, where the second layerexhibits a valence band energy and a conducting band energy duringoperation of the photodetector, wherein the relationship between thefirst and second layer valence and conduction band energies and thebarrier layer conduction and valence band energies facilitates minoritycarrier current flow while inhibiting majority carrier current flowbetween the first and second layers; and where the second layer includesa plurality of photo-absorbing layers; and a plurality ofstrain-compensating layers interspersed between the plurality ofphoto-absorbing layers, wherein the photo-absorbing layers are grownsubstantially lattice-mismatched to the barrier layer, and thestrain-compensating layers are interspersed between the photo-absorbinglayers so as to substantially compensate for a mechanical strain of thephoto-absorbing layers caused by the lattice-mismatched condition.
 71. Astrain-balanced photodetector with an extended wavelength, comprising: afirst layer disposed on a semiconductor substrate; a barrier layerlocated above the first layer; and a second layer located above thebarrier layer, wherein the first layer comprises a plurality ofphoto-absorbing layers; and a plurality of strain-compensating layersinterspersed between the plurality of photo-absorbing layers, whereinthe photo-absorbing layers are substantially lattice-mismatched to thesubstrate, and the strain-compensating layers are interspersed betweenthe photo-absorbing layers so as to substantially compensate for amechanical strain of the photo-absorbing layers caused by thelattice-mismatched condition; and wherein each of the plurality ofstrain-compensating layers has a thickness such that thestrain-compensating layers are substantially quantum-mechanicallytransparent.
 72. A strain-balanced photodetector with an extendedwavelength, comprising: a first layer disposed on a semiconductorsubstrate, where the first layer exhibits a valence band energy and aconducting band energy during operation of the photodetector; a barrierlayer located above the first layer, where the barrier layer has a bandenergy gap and associated conduction and valence band energies; and asecond layer located above the barrier layer, where the second layerexhibits a valence band energy and a conducting band energy duringoperation of the photodetector, wherein the relationship between thefirst and second layer valence and conduction band energies and thebarrier layer conduction and valence band energies facilitates minoritycarrier current flow while inhibiting majority carrier current flowbetween the first and second layers; and wherein the first layerincludes a plurality of photo-absorbing layers; and a plurality ofstrain-compensating layers interspersed between the plurality ofphoto-absorbing layers, and wherein the photo-absorbing layers aresubstantially lattice-mismatched to the substrate, and thestrain-compensating layers are interspersed between the photo-absorbinglayers so as to substantially compensate for a mechanical strain of thephoto-absorbing layers caused by the lattice-mismatched condition.